I accidentally shift-clicked my stack of fireworks while Enchanting, and the text that denoted its flight time was deleted. I further tested this by making a firework star and adding that to a firework and it lost the star effect too. This reddit post https://www.reddit.com/r/Minecraft/comments/er1bob/fireworks_and_enchanting_table/ shows what that looks like. They no longer stack with other fireworks after you shift-click them into the enchanting table.